Synthesis and reactivity of low valent main-group compounds

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Abstract

Low-valent, low-coordinate or multiply bonded main group compounds are of interest
for their reactivity. Properties usually observed for transition metals, such as small
molecule activation, are possible for these classes of main group compounds. The
development of the coordination chemistry of main group elements allows the
synthesis of new types of species, which could not be achieved before, by providing
electronic and kinetic stabilisation. In the following sections are presented reactivity
studies of silylenes, low-valent silicon species, and phosphaborenes (RP=BR’),
multiply bonded phosphorus-boron compounds.
Chapter I gives an overview of low-valent and low-coordinate silicon chemistry. Two
main types of silicon species can be distinguished in this area: silylenes and base-stabilised
silicon compounds. Silylenes (:SiR2) are heavier analogue of carbenes
(:CR2) and have simultaneously similar and different properties compared to
carbenes. Although less studied than carbenes, silylenes also have interesting
potential for coordination chemistry to transition metals or main group elements.
Base-stabilised silicon compounds have been known for more than a century,
however, only the recent use of N-heterocyclic carbene (NHC) ligands has allowed
isolation of new low coordinate silicon species, such as the carbene-stabilised
diatomic and triatomic silicon(0) clusters, which respectively feature two and three
silicon atoms in the formal oxidation state of zero.
Chapter II is a study of oxidative addition to an N-heterocyclic silylene 1,3-bis(diisopropylphenyl)-1,3-diaza-2-silacyclopent-4-en-2-ylidene, a heavier analogue
of N-heterocyclic carbene. An N-heterocyclic silylene was reacted with several main
group halides (SiI4, PCl3 and BBr3). Oxidative addition was observed in each case,
however, stability studies of the oxidative addition products show that only the product
from the reaction with SiI4 is isolable. The reactivity of this product was studied toward
reduction with alkali metals, reaction with bases, such as carbenes, organolithium
reagents and phosphines. The addition of an excess of the N-heterocyclic silylene
allows double oxidative addition from SiI4 to two N-heterocyclic silylenes. Finally, the
addition of an excess of N-heterocyclic silylenes to BBr3 allows the observation of a
relatively stable silylene-coordinated silylborane.
Chapter III covers the attempted synthesis of a new type of N-heterocyclic silylene,
which involves a bridgehead nitrogen atom, providing enhanced reactivity to this N-heterocyclic
silylene compared to classical examples. This chapter includes the full
synthesis and characterisation of the diamine ligand precursor N-1,3-
diisopropylphenyl-3-piperidinemethanamine. The study of its reactivity was then
carried out in order to insert silicon halide species into the structure, to reach the
targeted N-heterocyclic silylene by reduction. The different conditions, which were
tried on the ligand, did not successfully afford the targeted silane. The N-heterocyclic
silylene could not be achieved.
Chapter IV introduces the chemistry of mixed group III-V (13-15) compounds, which
are widely used in electronic devices. Their synthesis involves harsh conditions such
as chemical vapor deposition (CVD), metal-organic chemical vapor deposition
(MOCVD) or molecular beam epitaxy (MBE). Solution phase synthesis is a possible
route to these mixed group III-V compounds, which will allow the use of milder
conditions. Already achieved for some of them (GaAs, GaP, GaSb, InP, InSb), the
solution phase synthesis of lighter mixed group III-V compounds, such as boron-phosphide
(PB), is still elusive. The recent development of the chemistry of bas-estabilised
phosphaborenes RP=BR’(L), mixed compounds involving a phosphorus-boron
double bond, highlighted a possible way to make boron-phosphide through
solution phase preparation. The use of N-heterocyclic carbenes (L) could also allow
stabilisation of new allotropes of boron-phosphide PB(L).
Chapter V explores the chemistry of minimally substituted base-stabilised
phosphinoboranes [(Me3Si)2PBBr2(L)] (L = NHC) and phosphaborenes
[Me3SiP=BBr(L)] achieved by base-promoted abstraction of trimethylsilyl halide from
a phosphorus-borane adduct precursor [(Me3Si)3P→BBr3]. The functionalisation of
[(Me3Si)2PBBr2(L)] affords the [H2PBBr2(L)] and similar base-promoted
dehydrohalogenation allows the synthesis of the phosphaborene [HP=BBr(L)]. An
unsaturated NHC used in this chemistry showed limited stability to the conditions used
in the attempt to form base-stabilised boron-phosphide. The chemistry has been
reexplored using a new NHC, which is expected to be more stable and enable the
synthesis of base-stabilised boron-phosphide PB(L). The reactivity of
phosphaborenes has been explored and hydrogenation of [HP=BBr(L)] or
[(Me3Si)P=BBr](L) successfully gives the phosphinoboranes [H2PBHBr(L)] and
[(Me3Si)HPBHBr(L)]. Base-promoted dehydrohalogenation from [H2PBHBr(L)] allows
the observation of the base-stabilised parent diphosphadiboretane, [(HPBH)2(L)2].